Printed wiring boards or circuit boards are typically fabricated using an insulated layer of material with a conductive layer attached on each side. Circuitry is etched on the exterior conductive layer(s) using a combination of photo images and chemical etching to create the desired circuitry. Integrated circuits and many other electronic components may be added to these circuit boards and the assembled boards may be placed into packages. These circuit board packages may be used as sub assemblies and placed on other larger boards.
In order to attach these packages to a circuit board, some form of electrical connection must be made. Typical prior art package design uses a series of plated through holes around the exterior edges called castellations. The outside edges of the boards are machined to expose half of the castellation. The overall outside dimensions of these devices coincides with the end users product layout and is soldered onto their circuit card assembly. The half castellations provide the solder joint to mechanically and electrically connect the electronic packages to the next higher level assembly. Castellations of this style are common for low cost applications and have been in use for many years.
Problems associated with a castellation style termination are numerous for both original equipment manufacturers and their customers. Manufacturing problems include the presence of burrs from the machining process and from the process to singulate the boards from multi-board panels to produce the individual circuit card assembly. Burrs are unacceptable due to the potential of causing shorts from any loose conductive materials left on the printed circuit card assembly. Scoring and routing techniques are used to perform this depaneling process; both are inconsistent and rely on the individual operator's skill to repeat this operation consistently.
End users assembling electronic packages with end castellations typically stencil solder paste onto conductive pads and place the circuit boards with end castellation onto the solder paste. The solder paste is heated to sufficiently reflow into a smooth solder connection between the solder pads and the end castellations. A number of issues arise from this process. First, cleaning between the two printed circuit boards is difficult due to the zero clearance interfaces between the two printed circuit boards. Second, inspection of the reflowed solder paste between the assemblies is impossible due to the limited visual access at the interface joint. Third, removal of the part for troubleshooting and/or repair requires extreme heat directed at both the top and bottom circuit card assemblies in order to bring the temperature to an adequate level to reflow the solder. Many times these temperature extremes damage the solder joints or create open or short circuits to the internal components of the electronic packages.
U.S. Pat. No. 7,246,434 to Taylor et al. purports to describe providing an alternative attachment mechanism using standoffs integrally formed with a printed circuit board. The standoffs provide a gap between a printed circuit board module and the larger printed circuit board component to which the module is attached. However, the system described in the Taylor patent still relies on castellations to provide electrical connection, with all of the problems of castellations described above as well as an excessive amount of machining needed to remove material from the component under body to create the standoff.